The present invention relates to an improved process for the removal of acid gases, such as H.sub.2 S and CO.sub.2 from process gas and liquid streams by contact with amine solutions. More particularly, use of the present invention significantly reduces the amount of residual acid gas in the absorber overhead product stream. In addition, the process has the potential to significantly reduce the amount of extra stripper reboiler heat duty often needed to obtain adequate regeneration of the amine solution to enable an absorber to reduce the acid gas contents of a product stream to desired levels.
Acid gases (e.g., H.sub.2 S and CO.sub.2) are removed from process gas and liquid streams by contacting with an amine solution, typically aqueous alkanolamine solutions. Current product and regulatory requirements mandate reductions of the acid gas content of the product, particularly the sulfur contents. The expected trend is for continued reduction of sulfur content mandated by regulatory agencies. Such reductions can strain or exceed the design and operation capability of many amine treating systems. Also, increased production demands and/or processing of higher sulfur crudes by oil refineries or more sour natural gas can similarly strain the capacity of an amine system. Improvement in the removal of acid gas (H.sub.2 S primarily), may become critical to the ability of many plants to stay in compliance with federal and state regulatory limits on maximum sulfur contents of fuel gas and overall plant sulfur emissions.
For example, failure of a tail gas treating unit (TGTU) to reduce the sulfur content of the TGTU absorber off-gas to levels required to keep the stack gas sulfur content below maximum levels mandated by state and federal air quality regulations can result in having to shut down a plant until the sulfur content of the TGTU absorber off gas can again be reduced to acceptable levels.
Current practice employs a recycle process involving the counter-current contact and absorption of the acid gas by amine in a column, followed by thermal regeneration of the amine by counter-current flow of rich amine fed to the top of a stripping column and steam generated by reboiling the amine solution at the base of the stripping column, to reduce the acid gas content of the amine, followed by cooling and return to the absorber. Acid gases containing H.sub.2 S which are released in the stripping column are usually disposed of by incineration of small quantities or by conversion of larger quantities to elemental sulfur or sulfuric acid.
In the processing of gas and liquid streams, acid gas (e.g., H.sub.2 S and CO.sub.2) impurities are removed by contact with an amine solution. Amine solutions are used because of their relatively high absorption capacity of acid gases, and the relative ease with which acid gases can be stripped from the amine solutions by heat, typically at pressures of 25-35 psia. The regenerated amine solution is then cooled and recycled to the contactor again to absorb more acid gas. Thus, an amine solution is circulated around and around through the system, which typically consists of a contactor (or absorber), a stripper (or regenerator), and pumps, reboiler, coolers, heat exchangers, etc. An amine solution that contains large quantities of absorbed acid gas is called rich amine. When stripped of acid gas, it is called lean amine. The residual acid gas remaining in the lean amine solution is commonly called lean loading. Generally, the contactor is a counter-current flow tower wherein the lean amine is fed to the top of the contactor and flows from top down while the sour process gas or liquid stream is fed to the bottom of the contactor and flows from bottom to top. The stripper is also a counter-current flow tower, wherein amine flows from top to bottom and steam and acid gas vapors flow out the top. The primary purpose of the system is to reduce the acid gas content of a process stream to a specified level. The operation of the amine system is therefore driven by the need for acid gas removal in the contactor.
The use of counter-current flow design for the contactor intends that the leanest amine (amine that has the least acid gas content) contact the leanest process stream, to maximize the amount of acid gas removed from the process stream leaving the contactor. The degree to which this is realized depends on equipment, solvent type, process design, and process operating conditions. This includes hardware design for columns (including the selection of internal mass transfer devices) and operating conditions of temperature, pressure, flow rates, and acid gas loadings of both the lean and rich amine solutions.
The residual acid gas loadings of both H.sub.2 S and CO.sub.2 in the regenerated (lean) amine solutions are probably the major limiting factors affecting the ability of the amine system to take the acid gas content of the treated process stream to lower levels. For example, the partial pressures of the acid gases in the treated gas leaving the contactor cannot be reduced to levels below the acid gas partial pressures existing over the lean amine solution due to the residual acid gas loading. Therefore any significant reduction in residual acid gas loading of the lean amine will improve the degree of acid gas removal from the process stream leaving the contactor.
When the acid gas partial pressures exerted by the lean amine entering the contactor are too high to provide the required level of acid gas removal from the process stream, the amine stripper operation becomes the focus of attention. Improvement of the amine stripping operation to reduce the acid gas loading of the lean amine solution, will result in a lowering of the acid gas partial pressure over the lean amine entering the contactor. For example, either additional stripper reboiler heat duty to increase the amount of internal stripping steam generated or, where permissible, a reduction of the amine flow to the stripper, with the same reboiler duty, will result in better regeneration of the lean amine. The former increases immediate operating costs resulting from the increased heat duty (steam) provided while maintaining solution circulation rate and gross acid gas pickup capacity. The latter option of decreasing the flow of rich amine to the stripper will result in a lowering of sensible heat requirements, which in turn results in an increase of heat available to produce more internal stripping steam. This reduction in amine circulation rate however will result in higher acid gas loading of the rich amine from the contactor. If the partial pressures of the acid gases in the sour contactor feed stream are limiting in regard to the amount of acid gas the rich amine solution can absorb, the overhead process stream from the contactor will go sour (retain too much acid gas) regardless of how well the lean amine has been regenerated.
The prior art for obtaining a more complete regeneration of acid gas from a lean amine gas treating solution has typically involved increasing the stripper reboiler heat duty to increase the amount of steam stripping in the lower portion of the amine regenerators, or increasing the back-pressure of the amine regeneration process to increase the boiling point of the lean amine solution. The acid gases are not held by the amine as strongly at the higher temperature. The use of higher temperature however, typically results in more thermal decomposition and degradation of the amine. Thus, there remains a need for a method capable of providing leaner amine treating solutions that can further reduce the residual acid gas content of process streams being treated.